Adult periodontitis is disproportionally distributed among races, and has a significantly higher incidence in non- Hispanic blacks compared to non-Hispanic whites, even when co-variates such as diabetes and other health and social factors have been taken into account. Titers to the oral pathogen P. gingivalis are reported to be higher, as anticipated, in dentate subjects with, compared to those without periodontitis. Our laboratory was to first to report an intergeneric communication between Streptococcus cristatus and P. gingivalis. We identified a surface protein of S. cristatus, arginine deiminase (ArcA), as a signal molecule to which P. gingivalis responds by repressing the expression of the fimA gene, which codes for the major subunit protein of long fimbriae. The latter is required for bacterial colonization of P. gingivalis and host cell invasion. Therefore this project will test the hypothess that: a) S. cristatus serves as a beneficial bacterium and plays an important role in the prevention of P. gingivalis colonization of the oral cavity; and b) that the increased incidence of periodontitis in African-Americans may be due to either a decreased ratio of oral S. cristatus: P. gingivalis in these subjects compared to whites, or that the FimA genotypes may differ in blacks vs. whites. Our aims therefore are to: 1) determine the distribution of S. cristatus and P. gingivalis in dental plaques of those with moderate periodontitis, severe periodontitis and periodontal healthy individuals, and compare microbial profiles (especially S. cristatus and P. gingivalis), in Africa Americans versus Caucasians within each group; and 2) to examine the correlation of these prevalence of S. cristatus and P. gingivalis with different FimA genotypes. To achieve these aims, we will recruit three groups of subjects: those with a healthy periodontium, or with moderate versus severe periodontitis. Each group will be composed of adult African American and Caucasian males and females. Exclusion criteria will include diabetes, HIV-AIDs and other health criteria such as pregnancy. All subjects will receive a full-mouth examination of periodontal status and a medical history. Microbial samples from dental plaque will be collected. The prevalence of S. cristatus and P. gingivalis will be determined by qPCR analysis, as will the prevalence of different FimA genotypes. We anticipate that the proposed studies will provide important information regarding the pathogenesis of periodontitis, and the microbial biofilms found in these two populations and will pave the way for extended studies in future, focused in the longer term on the development of appropriate interventional strategies targeting at eliminating of P. gingivalis colonization.